1. Field of the Disclosure
The present disclosure relates to a capacitive input device used in various electronic units, and more particularly to a capacitive input device having a curved surface shape.
2. Description of the Related Art
Recently, capacitive input devices have come into widespread use that detect a slight change in capacitance at a portion that a user has lightly touched with a fingertip, convert a position and a direction of motion to electric signals, and output the converted signals. Capacitive input devices of this type are advantageous in that switches and other mechanical parts can be eliminated and these capacitive input devices are thereby superior in design but are disadvantageous in that vacuum deposition is needed because an indium tin oxide (ITO) film is used as a transparent electrode for use in detection. Therefore, the shapes of these capacitive input devices are limited essentially to a simple two-dimensional shape (planar shape) from an industrial viewpoint, placing a limit on portions of products to which capacitive input devices are applied.
In view of the above disadvantages, Japanese Unexamined Patent Application Publication No. 2010-267607 proposes capacitive sensors (capacitive input devices) 900 that can be attached to a portion with a curved surface. As conventional examples, FIGS. 9A and 9B illustrate the capacitive sensors 900 proposed in Japanese Unexamined Patent Application Publication No. 2010-267607; FIG. 9A schematically illustrates a capacitive sensor 900A in embodiment A and FIG. 9B schematically illustrates a capacitive sensor 900B in embodiment B.
The capacitive sensor 900A in FIG. 9A and capacitive sensor 900B in FIG. 9B each include a film base 901, which is three-dimensionally formed, a decorative layer 910, which is integrally formed on the film base 901, a conductive circuit pattern layer 920 that detects a change in capacitance, a supporting body 940 that supports the film base 901, and a protective layer 930A that suppresses conductive ink in the circuit pattern layer 920 from exuding. In addition, the capacitive sensor 900A in FIG. 9A includes a protective layer 935 that prevents the surface of the decorative layer 910 from being scratched or worn, and the capacitive sensor 900B in FIG. 9B includes an insulative resist layer 925. When a finger EN of the user touches a surface in a three-dimensional shape, the capacitive sensor 900 (900A or 900B) detects, in the circuit pattern layer 920, a slight change in capacitance at the portion that the tip of the finger FN has touched.
With the capacitive sensors 900 (900A and 900B), the circuit pattern layer 920 includes a first circuit pattern layer 923 and a second circuit pattern layer 924 to improve detection precision of the capacitive sensors 900 (900A and 900B) and diversify their structures. Furthermore, the circuit pattern layer 920 is formed by using a conductive ink including a silver paste, a carbon ink, a conductive polymer (Seplegyda from Shin-Etsu Polymer Co., Ltd.), and the like without using an indium tin oxide (ITO) film, so the circuit pattern layer 920 can be formed on the film base 901, which is three-dimensionally formed, and the supporting body 940.
However, the property of the conductive polymer (conductive macromolecules) used in the conventional examples is that when the layer of the conductive polymer absorbs moisture, the resistance of the layer is increased. Therefore, there has been a risk that if absorbed moisture is not removed, detection precision is lowered. Particularly, in the structures in the conventional examples, the second circuit pattern layer 924 in the capacitive sensor 900A and the circuit pattern layer 920 (including the first circuit pattern layer 923 and second circuit pattern layer 924) in the capacitive sensor 900B are structured so as to be interposed between the film base 901 and the supporting body 940. This has been problematic in that once moisture penetrates into the film base 901 or supporting body 940 and is absorbed by the circuit pattern layer 920 (including the first circuit pattern layer 923 and second circuit pattern layer 924), the moisture is not easily removed. Therefore, there has been a risk that after the capacitive sensors 900 in the conventional examples have been exposed to a humid environment, detection precision is lowered and the capacitive sensors 900 does not easily recover from the lowered detection precision.